1. Field of the Invention
The invention is concerned with movable vane assemblies of the type used in a gas turbine engine or a compressor for interaction with a gas stream. More particularly, the invention is concerned with an improved temperature independent means for indicating the rotational position of a plurality of radially aligned vanes which are used to vary the effective cross sectional area of an annular passage through which gas flows in a gas turbine engine or compressor.
2. Prior Art
Variable area nozzles are utilized in gas turbine engines to improve the efficiency over relatively wide ranges of motor speeds. In such nozzles, it is necessary to accurately position each of plurality of movable vanes and to maintain them in selected positions during the operation of the turbine motor. Such accuracy of positioning is difficult to maintain due to, for example, excessive tolerances arriving at a drive arrangement for positioning the vanes, to distortion caused by the flow of hot gases through the nozzles, and to distortion caused by imbalanced loading on the actuator parts.
In the prior art, one arrangement for controlling movable nozzle vanes employs a ring gear arranged about the nozzle vanes for simultaneous positioning thereof. Exemplary of such prior art systems are those disclosed in U.S. Pat. Nos. 3,252,686 to Chadwick; 3,383,090 to McLean; and 3,376,028 to Williamson. Each of these arrangements utilize single actuator jacks for rotating a ring member which causes a simultaneous actuation of a plurality of vanes through connecting means. Such systems, due to the single point force application from the actuator jack, cause an imbalance of loading on the ring member which can lead to distortion of the parts and inaccuracy in nozzle placement.
Other prior art devices utilize a plurality of separate actuator jacks connected to various points along a ring gear to position the same. Again, an imbalance of forces on the ring gear is occasioned by an unequal or imprecise movement of the multiple jacks. Copending U.S. application Ser. No. 609,764, now U.S. Pat. No. 4,003,675, commonly assigned herewith and filed Sept. 2, 1975, discloses a vane positioning system which comprises a ring gear acted upon at two diametrically-opposed peripheral points by dual bell crank linkages which in turn are acted upon by a single double-acting hydraulic motor. The motor is free to move between the two bell crank connections and they thus exert equal and opposite forces upon each of the bell crank linkages to produce a balanced force upon the ring gear. Rotation of the ring gear causes simultaneous adjustment of a plurality of movable nozzle vanes.
The present invention is directed to an improved temperature independent means for indicating the rotational position of such nozzle vanes as are motivated by such dual bell crank linkages as are disclosed in U.S. patent application Ser. No. 609,764 and other related linkages.